Observations on Role of Free Sulfhydryl Groups in Prothrombin Activation. ∗ †

Abstract

Summary

(1) One-stage prothrombin determinations in human plasma are only slightly affected by a wide range of GSH concentrations whereas relatively low levels of GSSG result in a marked lengthening of the prothrombin times. (2) Incubation of human plasma with either GSH or GSSG prior to initiation of prothrombin activation produces inhibitions of thrombin formation, the same degree of inhibition being observed with considerably lower concentrations of GSSG than of GSH. (3) A study of the precise point of action of the two forms of glutathione reveals that at least 75% of their over-all effects are due to reactions with clotting factors occurring during the pre-incubation period. (4) Pre-incubation of human plasma has resulted in an immediate decrease in the non-protein sulfhydryl concentration of the mixture over and above that accountable for by reaction with prothrombin. A concomitant increase in the non-protein disulfide level of the system appears to indicate the formation of some GSSG and supports the view that GSH inhibits clotting not only by reducing prothrombin disulfide linkages, but also by giving rise to some GSSG which, as has been shown subsequently, has marked inhibitory powers of its own. (5) Data have been presented which support the hypothesis that GSSG as well as other SH-oxidizing and -blocking agents react with at least one prothrombin conversion accelerator which is present in higher concentrations in dog plasma than in human plasma. This, in turn, implies the essential nature of free SH groups on such an accelerator and establishes their importance in the blood clotting mechanism.(6) Possible explanations and implications of these findings have been discussed.